Flexible circuits are commonly used in interconnection systems because of their ease of assembly to like circuits or to rigid circuit substrates such as printed circuit boards. A flexible circuit is typically connected to another circuit by forced contacts between electrical contact projections on the flexible circuit and conductive elements, such as traces, on a printed circuit board. Because of manufacturing tolerance variations in the heights of the contact projections, as much as 200 grams of force per projection may be needed to ensure reliable contact closure. For a circuit having at least 260 contact projections, this translates into a 52 kilogram force on the circuit, resulting in force-related material and contact failures in assembled units.
Flexible circuits with coplanar contact projections require less force to ensure reliable contact between the projections and conductive elements. However, the successful manufacture of coplanar contact projections has been difficult to achieve; attempts have ranged from photolithographic processes to mechanical extrusion-based processes.
For example, a photolithographic approach involves conventional deposition and etching processes to produce base contacts on a flexible circuit substrate. Copper is then electrodeposited on the base contacts to form the contact projections. This method may further include the use of a thicker base contact foil from which the contact projections are directly formed, thus eliminating the need for electroplating. Yet, thickness variations in the flexible circuit substrate typically alter the height, and thus coplanarity, of the contact projections.
In an alternate approach, a mechanical die extrudes the flexible circuit through unbounded holes to produce the contact projections. Specifically, the base contacts of the flexible circuit are electroplated with thick layers of copper. The circuit is placed in a press over a die plate that has holes corresponding to the desired contact projections. Extrusion pins are then forced down on the circuit to extrude the flexible circuit substrate material through the holes in the die plate and thereby form the projections. As above, the thickness variations in the flexible circuit substrate typically affect the coplanarity of the contact projections, since the projections are formed by means of unbounded holes. That is, the geometries of the the die plate holes and the die mating parts, i.e. extrusion pins, together with the force applied by the punch press determine the shape and planarity of the extruded contact projections.